FIG. 5 shows an outline configuration of a lubricating system around an air intake device and a valve gear device as to a general gas engine.
In FIG. 5, the engine that is depicted with the numeral 100 is a four-stroke cycle gas-engine provided with a spark plug 113; thereby, the engine comprises a cylinder liner 102a in which a piston (not shown in FIG. 5) is guided so as to perform reciprocating movements, and a combustion chamber 101 that is formed below the undersurface of a cylinder head 106a, over the top surface of the piston, inside the inner surface of the cylinder liner 102a. 
Further, the engine 100 comprises an intake-air port 103 that communicate with the combustion chamber 101, an intake-air valve 104 that opens and closes so as to control the intake air flow through the air intake-air port 103 into the combustion chamber 101, an exhaust gas port 106 that communicate with the combustion chamber 101, and an exhaust (gas) valve 107 that opens and closes so as to control the exhaust gas flow from the combustion chamber 101 to the exhaust gas port 106.
An intake air pressure-regulating device (a zero-governor) 112 regulates the pressure of the fuel gas; the fuel gas under the regulated pressure is supplied into a gas mixer (an air fuel-gas mixer) 110 through a fuel gas supply pipe 110a. In the air fuel-gas mixer 110, the fuel gas supplied through a fuel gas supply pipe 110a is mixed with the air supplied through an air (inducing) pipe 103b; thus, air fuel-gas mixture is formed; the formed air fuel-gas mixture is supplied to each intake-air port 103 of the engine.
Further, the air fuel-gas mixture reaches each intake air valve 104 through the corresponding intake-air port 103, and is charged into the combustion chamber 101 while the intake air valve is being opened.
As described above, in the gas mixer 110, mixing the fuel gas with the air supplied through an air (inducing) pipe 103b forms the air fuel-gas mixture that is supplied to the intake-air port 103.
The air fuel-gas mixture flows toward the intake air valve 104 from an intake air pipe 103a through the intake-air port 103, and is charged into the combustion chamber 101 while the intake air valve is being opened. Further, by use of the spark plug 113, the air fuel-gas mixture charged in the combustion chamber 101 ignites; thereby, the spark plug is installed in the cylinder so that the tip part (the spark gap) of the spark plug borders the dead space of the combustion chamber 101.
On the cylinder head 106b, a (cylinder) head cover 24 is placed; the head cover 24 is fastened to the cylinder head with a plurality of bolts; and, in the head cover 24, a valve gear room (a space) 6 is formed so that a partition 6y divides the valve gear room 6 into an upper room 6v and a lower room 6u. Further, the partition 6y is provided with a reed valve 24s through which only one-way oil mist flow (blow-by gas flow), namely the oil mist flow from the lower room 6u to the upper room 6v, is allowed.
The blow-by gas with oil mist streams into the lower room 6u through a blow-by gas entrance pipe 2, for instance, from a space inside the cylinder block of the engine; such blow-by gas flows into the upper room 6v through a reed valve 24 from the lower room 6u. In the next place, the blow-by gas is sent into an air cleaner 25 through a blow-by gas discharge pipe 3 from the upper room 6v. 
Conventionally, there are many blow-by gas discharging means (systems) are shown.
According to the patent reference 1 (JP 2722120), the blow-by gas in the valve gear room 3 is returned back to the air inlet side of the gas mixer 8 through the breathing room 5 and the breathing pipe 12.
Further, in the patent reference 2 (JP 3962477), the blow-by gas in the valve gear room 102 returned back to the inlet side of the carburetor 111 through the breathing room 109 and the breathing pipe 132. Hereby, the numerals quoted are those quoted in the references 1 and 2 (not the numerals in the attached drawings).
In a case of a gas engine in comparison with a gasoline, the wear of the intake air valve (stem) guide as well as the exhaust gas valve (stem) guide becomes larger; further, the wear of the intake air valve seat as well as the exhaust gas valve seat becomes larger. A possible major reason is that the gasoline components in the air fuel mixture work as a lubricant in a case of a gasoline engine; on the contrary, the lubrication effect of the fuel in the air fuel mixture cannot be expected in a case of a gas engine.
Thus, in the gas engine as depicted in FIG. 5, there is a problem that the wear is prone to increase in relation to the contact area between the intake air valve cone 104 and the valve seat 115, between the intake air valve stem 104 and the valve stem guide 117, between the exhaust gas valve cone 107 and the valve seat 115, and between the exhaust gas valve stem 107 and the valve stem guide 117.